Modern refrigeration technology, as a science, was developed in the mid and late 19th century, before that, tracing back to the ancestors of the human being, people learned very long ago to make use of cold and make simple manual refrigeration, and using cellars and cold store and cooling a store room with spring water have a history of 5000 years.
The 20th century saw greater development of refrigeration technologies: the household refrigerator was born in 1910, and was marketed as a commodity in the United States in 1917. In 1930, the appearance of Freon refrigerating media and the use of Freon refrigerators brought new transformation to the refrigeration technologies. In the 1970s, massive researches were conducted on mixed media, and co-boiling mixed media were used, ushering a new road for the development of steam compression refrigerators. Refrigeration technologies have developed today from food preservation and temperature conditioning in given spaces to all sectors of national economy, becoming more closely linked with the daily life of people:
1. Commerce
Main applications of refrigeration technologies are cold processing, storage and transport of perishable food (such as fish, meat, eggs, fruits and vegetables), to reduce food loss in production and distribution and ensure balanced marketing over all seasons. In the modern food industry, a complete cold energy chain has been formed from food production, storage, transport and sales. Refrigeration apparatus used include refrigerating warehouses, refrigerating vehicles, refrigerating vessels and refrigerating trains and so on. In addition, refrigerating cabinets for commodities, various cold beverage equipment and commodity display cabinets with refrigerating equipment are used in food retail shops, canteens and restaurants.
2. Cooling and Air Conditioning
They include air conditioners for comfort in life, installed in hotels, theaters, metros, big public buildings, vehicles, aircraft cabins, offices and residences and so on, provide a comfortable living and working environment for people, not only good to physical and psychological health, but also increasing production and working efficiency.
3. Industrial Production
In machine building, cryogenic treatment of steel (−70° C.˜−90° C.) can change its metallographic structure, turn the Austenite into Martensite and increase the hardness and strength of steel, and in machine assembling, low temperature can be used to realize interference fit easily. In the chemical industry, refrigeration can help gas liquefaction and separate mixed gases, and carry away reaction heat in chemical reactions. Refrigeration is required in salt crystallization, lubricating oil degreasing, petroleum cracking, rubber and resin synthesizing, fuel and fertilizer production, and also in the liquefaction, storage and transport of natural gas. In iron and steel industry, air is first dehumidified by refrigeration before being blown into a blast furnace, to reduce coking ratio and ensure the quality of molten iron, normally, a big blast furnace needs cold quantity of several megawatts.
4. Agriculture and Animal Husbandry
Refrigeration is used for cryogenic processing of crop seeds, creating artificial climatron for seedling cultivation, and reserving seminal fluid of fine breeds for artificial breed mating.
5. Project Construction
Refrigeration can be used to realize frozen excavation. In digging mines and tunnels and building river dams, or in tunneling in swamp and stratum with sand and water, frozen earth process can be used to prevent collapse in working area and ensure construction safety. In concrete batching, ice can be used in place of water, to compensate for the cement solidifying reaction heat by the heat absorbed by melting ice, to build big single-column concrete members, and this can effectively avoid internal stress, cracks and other defects in large structural members due to insufficient heat dissipation.
6. National Defence Industry
For engines, vehicles, tanks, artilleries and other conventional weapons to be used in high and cold areas, environmental simulation test should be done on their performance; for control instruments in aircrafts, rockets and missiles, performance test is required by simulating the cryogenic conditions at high altitude on ground, and all these require refrigeration to provide the environmental conditions for tests. Refrigeration is also required in the control of atomic energy reactors.
7. Medical Service and Public Health
Refrigeration is required in cryosurgery, such as operation on heart, excision of tumor, cataract and tonsil, skin and eyeball transplanting and hypothermic anesthesia. In addition to storage of vaccines and drugs at low temperature, blood and skim are also stored with freezing vacuum drying method.
Furthermore, refrigerating technology also has important applications in cutting-edge sciences such as micro-electronic technology, energy, new type raw materials, exploitation of space and biological technology.
In summary, refrigerating methods can be classified into two categories: those by inputting work and those by inputting heat. Vapor compression refrigeration and thermoelectric refrigeration belong to the former, and absorption refrigeration, vapor jetting refrigeration, and adsorption refrigeration belong to the latter.
Researches on traditional refrigerating technologies can be summarized as the following three aspects:
1) Researches on methods to obtain low temperature, the relevant mechanisms and the corresponding refrigerating cycles, and thermodynamic analysis and calculation of refrigerating cycles.
2) Researches on the properties of refrigerants, to provide refrigerators with media having satisfactory performance. Mechanical refrigeration can be realized by changing the thermodynamic status of refrigerant, therefore, the thermal physical property of refrigerant is the basis for cycle analysis and calculation. Furthermore, for the practical application of refrigerants, their general physical and chemical properties must also be well known.
3) Researches on the machinery and technical equipment required to realize refrigerating cycles, their working principles, performance analysis, structural design and calculation, and the flow organization and system associated calculation for various refrigeration apparatus. In addition, there are questions about thermal insulation and automation of refrigerating equipment and so on.
The first two aspects mentioned above constitute the theoretical foundation of refrigeration, i.e. the researches on traditional refrigerating principles, and the third aspects is about the specific machines, equipment and apparatus.
The traditional refrigerating theory is mainly based on thermodynamics, i.e. Carnot reverse cycle of identical temperature difference is used to analyze the refrigerating cycle process, the economic indicator of the refrigerating cycle is the refrigeration coefficient, or the ratio of obtained gain to the cost of consumption, and also, of all refrigerating cycles between atmospheric environment with temperature of T0 and low temperature heat source with temperature of Tc (such as refrigeration store), the reverse Carnot cycle has the highest refrigeration coefficient:
                              ɛ          c                =                                            (              COP              )                                      R              ,              C                                =                                                    q                2                                            w                0                                      =                                          T                c                                                              T                  0                                -                                  T                  c                                                                                        (        1        )            
In the formula above, εc is the refrigeration coefficient, q2 refrigerating capacity of the cycle, and w0 the net work consumed by the cycle.
In fact, in his thesis “Reflections on the Motive Power of Heat”, Carnot concluded that “of all heat engines working between two constant temperature heat sources of different temperatures, the reversible heat engine has the highest efficiency.” This was later referred to as the Carnot theorem, after rearranging with the ideal gas state equation, the thermal efficiency of Carnot cycle obtained is:
                              η          c                =                  1          -                                    T              2                                      T              1                                                          (        2        )            
In Formula (2), temperature T1 of the high temperature heat source and temperature T2 of low temperature heat source are both higher than the atmosphere ambient temperature T0, and the following important conclusions can be obtained:
1) The thermal efficiency of Carnot cycle only depends on the temperature of high temperature heat source and low temperature heat source, or the temperature at which the media absorbs heat and release heat, therefore the thermal efficiency can be increased by increasing T1 and T2.
2) The thermal efficiency of Carnot cycle can only be less than 1, and can never be equal to 1, because it is not possible to realize T1=∞ or T2=0. This means that a cyclic engine, even under an ideal condition, cannot convert all thermal energy into mechanical energy, of course, it is even less possible that the thermal efficiency is greater than 1.
3) When T1=T2, the thermal efficiency of the cycle is equal to 0, it indicates that in a system of balanced temperature, it is not possible to convert heat energy into mechanical energy, heat energy can produce power only with a certain temperature difference as a thermodynamic condition, therefore it has verified that it is not possible to build a machine to make continuous power with a single heat source, or the perpetual motion machine of the second kind does not exist.
4) Carnot cycle and its thermal efficiency formula are of important significance in the development of thermodynamics. First, it laid the theoretical foundation for the second law of thermodynamics; secondly, the research of Carnot cycle made clear the direction to raise the efficiency of various heat power engines, i.e. increasing the heat absorbing temperature of media and lowering the heat release temperature of media as much as possible, so that the heat is release at the lowest temperature that can be naturally obtained, or at the atmospheric temperature. The method mentioned in Carnot cycle to increase the gas heat absorbing temperature by adiabatic compression is still a general practice in heat engines with gas as media today.
5) The limit point of Carnot cycle is atmospheric ambient temperature, and for refrigerating process cycles below ambient temperature, Carnot cycle has provided no definite answer.
Because of the incompleteness of refrigeration coefficient, many scholars at home and abroad conducted research on it, and proposed methods to further improve it. In “Research on Energy Efficiency Standard of Refrigerating and Heat Pump Products and Analysis of Consummating Degree of Cyclic Thermodynamics”, Ma Yitai et al, in conjunction with the analysis of introduction of the irreversible process of heat transfer with temperature difference into heat cycle by Curzon and Ahlborn and the enlightenment from the finite time thermodynamics created on it, as well as the CA cycle efficiency, proposed the consummating degree of thermodynamics of CA normal circulation, advancing to a certain extent the energy efficiency research on the refrigerating and heat pump products.
However, the basic theory of thermodynamics cannot make simple, clear and intuitional explanation of the refrigerating cycle. Einstein commented the classical thermodynamics this way: “A theory will give deeper impression to the people with simpler prerequisite, more involvement and wider scope of application.” In the theoretical interpretation in the refrigeration field, this point should be inherited and carried forward.
Therefore, it has become a difficult issue in the research of refrigeration technical field to really find the theoretical foundation for the refrigerating cycle, develop new refrigerating cycle apparatus on this theoretical foundation and apply them in practice, and effectively reduce the energy consumption.